Fuel injection systems operate by energizing coils and using those coils to move an electromagnet that opens a valve, allowing pressurized fuel to enter the cylinders of an engine. The coils in fuel injectors tend to have a large amount of inductance, causing a delay between the time voltage is applied to the coil and the time that the coil has sufficient current flow (e.g., 20 A) to actually begin fuel injection. The actual amount of the delay depends both on the inductance of the coil and the amount of voltage applied to the coil. The time delay between the leading edge of the forward pulse and the time the load current reaches the desired level is called the “ramp time.”
If the voltage differs from a nominal voltage, the amount of time required for the load current in the coil to ramp up to the desired level will change as a result. For example, if the voltage applied to the coil is lower than the nominal voltage, the load current will increase more slowly than expected. Similarly, if the voltage is higher than the nominal voltage, the load current will increase more quickly. These changes can cause the load current to open the valve at a time other than an expected time calculated from the nominal voltage, making it difficult to maintain precise timing over fuel injector activation.
The inductance in the coil may also vary from a nominal inductance, further changing the actual time in which the load current reaches its desired level. Variations in the coil inductance also makes precise timing of fuel injection difficult. Although voltage regulators can be used to stabilize the voltage applied to the coil, voltage regulators are expensive and add complexity to the fuel injection system.
There is a desire for a system that can compensate for changes in the voltage and the coil inductance to ensure that the load current reaches the desired level at a desired time accurately.